1. Technical Field
The present invention relates generally to performing updates of video frames in response to received bit errors and, more particularly, but not by way of limitation, to performing video fast updates in video telephony services in Third Generation networks.
2. History of Related Art
An important application used in Third Generation (“3G”) networks is Video Telephony services (“VT”). VT typically uses a transparent 64 kb/s bearer with an average bit error rate of approximately 10−4-10−3. A higher bit error rate means that a wireless network can handle more simultaneous VT calls; however, a higher bit error rate often causes video-transmission problems.
An encoded video bit stream is very sensitive to errors. The error sensitivity of the video bit stream is primarily due to heavy use of prediction from previous video frames and the use of Variable Length Coding, which can easily get out of synchronization if a bit error is introduced into the bit stream. Many mobile terminals include video decoders that employ error concealment functionality to hide the effects of corrupted bit streams. The video encoders are typically also configured to produce resilient bit streams that are fairly robust to errors.
In cases when there are too many errors in the bit stream, error concealment employed by the mobile terminal may not be sufficient to effectively conceal the errors. The video decoder can then inform the video encoder to refresh the image by encoding a video frame without prediction from previous images. A video frame without prediction is referred to as an intra frame and is much larger than a typical frame; therefore, intra frames should not be sent unnecessarily. The video decoder informs the video encoder to refresh the image by sending a Video Fast Update (“VFU”) message in accordance with the H.245 protocol to the video encoder.
Some mobile terminals send VFU requests at regular time intervals, meaning that complete intra frames are sometimes transferred even though no bit error has occurred. As a result, regular image freezes often occur in the received video, since intra frames take significantly longer to transmit than frames that utilize prediction, which are referred to as inter frames.
Other mobile terminals send VFU requests responsive to detection of a bit stream error. The VFU requests may be sent either by an H.223 demultiplexer or by an H.263/MPEG-4 video decoder of the mobile terminal. In this approach, an intra frame request is sent even when the detected error is so small that the error could be readily handled by the error concealment functionality of the video decoder. Thus, intra frames are sometimes unnecessarily transmitted.
Another solution, for updates of only parts of the image, is described in H.263 Appendix I. The solution described in H.263 Appendix I requires substantial changes in both the video encoder and updates to the Third Generation Partnership Project (“3GPP”) recommendations for video telephony.